1. Diffractive Studies at the Tevatron
Gregory R. Snow
University of Nebraska
(For the D0 and CDF Collaborations)
Introduction
Hard Color Singlet Exchange (CSE)
(forward jet - gap - forward jet)
Hard Single Diffraction
(forward gap - opposite dijets or W)
Double Pomeron Exchange Signatures
(forward gap - jets - forward gap)
Improvements for Tevatron Run II
Conclusions
CMS Collaboration Meeting
March 8, 2001
Workshop on Forward Physics and
Luminosity Determination at the LHC
Helsinki, Finland
November 2, 2000
ICHEP98
Vancouver, BC, Canada
July , 1998
International Europhysics Conference
Tampere, Finland
July , 1999

2. University of Nebraska (For the D0 Collaboration)
D0 Run I Diffraction Results
Gregory R. Snow
University of Nebraska
(For the D0 Collaboration)
Introduction
Hard Color Singlet Exchange (CSE)
(forward jet - gap - forward jet)
Hard Single Diffraction
(forward gap - opposite dijets or W)
Double Pomeron Exchange Signatures
(forward gap - jets - forward gap)
Diffractive W and Z Production
(W/Z plus gap)
Conclusions
New-ish
Nice result,
need to publish
New
Small-x Workshop
Fermilab
September , 2003

3. So why am I here?
Dino was my Ph.D. thesis advisor at RockU, somehow
diffraction gets in your blood
Oversaw some of this work as D0 Run I QCD co-convener
(with Schellman, Brandt, Elvira) and Editorial
Board member/chair
I have great appreciation for Andrew Brandt’s invention
and leadership of the D0 diffraction effort
Expanded D0’s Run I physics menu
Several unique Ph.D. theses and publications resulted
Run I pioneering work has led to Run II Forward
Proton Detector – more comprehensive studies
to come (see upcoming talks)
I said “yes” to this talk

4. Conclusion before the talk
So you’re at a cocktail party, and someone asks you:
“What fraction of (insert your favorite QCD dijet, W/Z,
direct photon, heavy flavor process here) events are
accompanied by a striking rapidity gap in the calorimeter?”
Your answer: “About ONE PERCENT. Come here often?”

5. QCD Physics from DØ
QCD publications represent 25% of the 125 papers published by
D from its Run 1 data sample (100 pb-1)
Jet physics
“Transverse Energy Distributions within Jets at 1.8 TeV”
“Studies of Topological Distributions of Inclusive Three- and Four-Jet Events at 1800 GeV”
“Measurement of Dijet Angular Distributions and Search for Quark Compositeness”
“Determination of the Absolute Jet Energy Scale in the D Calorimeters”
“The Dijet Mass Spectrum and a Search for Quark Compositeness at 1.8 TeV”
“The Inclusive Jet Cross Section in Collisions at 1.8 TeV”
“Limits on Quark Compositeness from High Energy Jets in Collisions at 1800 GeV”
“The ratio of Jet Cross Sections at 630 GeV and 1800 GeV”
“Ratios of Multijet Cross Sections at 1800 GeV”
“High-pT Jets at 630 and 1800 GeV”
Direct photon physics
“The Isolated Photon Cross Section in the Central and Forward Rapidity Region at 1.8 TeV”
“The Isolated Photon Cross Section at 1.8 TeV”
QCD with W and Z
“A Study of the Strong Coupling Constant Using W + Jets Processes”
“Measurement of the inclusive differential cross section for Z bosons as a function of transverse momentum produced at 1.8 TeV”
“Evidence of Color Coherence Effects in W+Jets Events at 1.8 TeV”
“Differential production cross section of Z bosons as a function of transverse momentum at 1.8 TeV”
“Differential Cross Section for W Boson Production as a Function of Transverse Momentum in Collisions at 1.8 TeV
Rapidity gaps, hard diffraction, BFKL dynamics
“Rapidity Gaps between Jets in Collisions at 1.8 TeV”
“The Azimuthal Decorrelation of Jets Widely Separated in Rapidity”
“Jet Production via Strongly-Interacting Color-Singlet Exchange”
“Color Coherent Radiation in Multijet Events from Collisions at 1.8 TeV”
“Probing Hard Color-Singlet Exchange at 630 Gev and 1800 GeV”
“Hard Single Diffraction in Collisions at 630 and 1800 GeV”
“Probing BFKL Dynamics in Dijet Cross Section at Large Rapidity Intervals at 1800 and 630 GeV”
“Observation of Diffractively Produced W and Z Bosons in Collisions at 1800 GeV”

6. D0 Hard Diffraction Studies
Understand the Pomeron via ……
Or W/Z

7. Calorimeter tower thresholds used in rapidity gap analyses
DØ Detector (Run I)
(nL0 = # tiles in L0 detector with signal
2.3 < |h| < 4.3)
Calorimeter tower thresholds
used in rapidity gap analyses
beam
Central Gaps
EM Calorimeter
ET > 200 MeV
|h| < 1.0
Forward Gaps
E > 150 MeV
2.0 < || < 4.1
Had. Calorimeter
E > 500 MeV 3.2 < || < 5.2
L0 Detector
End Calorimeter
Central Calorimeter
EM Calorimeter
Central Drift Chamber
(ntrk = # charged tracks with |h| < 1.0)
Hadronic Calorimeter
(ncal = # cal towers with energy above threshold)
   = 0.1  0.1
Three different rapgap tags

8. Central Gaps between Dijets
Two forward jets with |h | > 1.9, Dh > 4.0
and rapidity gap in |h | < 1.0
Signature for color singlet exchange
Study gap fraction fS as function of
Center of mass energy (1800, 630)
Jet ET Dh
Compare with Monte Carlo color singlet models
Low energy run (630 GeV) very illuminating
No tracks or
cal energy
jet h f
|h | < 1.0

9. DØ Central Gaps between Dijets
Nevents
ncal
2-D multiplicity (ncal vs. ntrk) between two leading
jets (ET > 12) for (a) 1800 GeV and (b) 630 GeV
Redundant detectors important to extract
rapidity gap signal
Dijet events with one interaction per bunch crossing
selected using multiple interaction flag
fS = (Ndata- Nfit)/Ntotal
Rapidity gap fraction
Negative binomial
fit to “QCD”
multiplicity

10. Example of comparison to different
DØ Central Gaps between Dijets
Fraction of events with central gap
fS630 =  0.09  0.37 %
fS1800 = 0.54  0.06  0.16 %
(stat) (sys)
Sys. error dominated by background
fit uncertainties
Ratio (630/1800)
fS630/fS1800 = 3.4  1.2
Example of comparison to different
color singlet models
s = 1800 GeV
In the soft-color scenario, one can
extract a ratio of gap survival
probabilities {
Gap fraction vs. second-highest jet ET
1.5  0.1
Data consistent with a soft color rearrangement
model preferring initial quark states, inconsistent
with two-gluon, photon, or U(1) models

11. Hard Single Diffraction
Measure multiplicity here
Measure minimum multiplicity here h
Jet ET’s > 12, 15 GeV
Forward jets
Central jets
Two topologies h
Gap fractions (central and forward) at s = 630 and 1800 GeV
Single diffractive x distributions
Comparisons with Monte Carlo to investigate Pomeron structure
Physics Letters B531, 52 (2002)

12. or Measure Multiplicity here -4.0 -1.6 -1.0 h 1.0 3.0 5.2
s = 1800 GeV
s = 630 GeV
2-D Multiplicity
distributions
Central jets
Forward jets
L0 scintillator
tiles used now
Forward jets
Central jets

13. Hard Single Diffraction
1800 Forward Jets
Event Characteristics
Fewer jets in diffractive
events
Jets are narrower and
more back-to-back
(Diffractive events have
less overall radiation)
Gap fraction has little
dependence on average
jet ET
Solid lines show show HSD candidate events
Dashed lines show non-diffractive events

14. Hard Single Diffraction
Data
Signal
Measured Gap
Data Set Fraction
1800 Forward Jets (0.65  0.04)%
1800 Central Jets (0.22  0.05)%
630 Forward Jets (1.19  0.08)%
630 Central Jets (0.90  0.06)%
Data Sample Ratio
630/1800 Forward Jets  0.2
630/1800 Central Jets  1.0
1800 Fwd/Cent Jets  0.7
630 Fwd/Cent Jets  0.1
Gap Fraction
# diffractive Dijet Events / # All Dijets
Background
Signal extraction
Forward 1800 GeV example
Signal: 2D falling exponential
Background: 4 parameter
polynomial surface
Lessons
Forward Jets Gap Fraction > Central Jets Gap Fraction
630 GeV Gap Fraction > 1800 GeV Gap Fraction
Publication compares data with POMPYT M.C. using different quark and gluon structures.
This data described well by a Pomeron composed dominantly of quarks, or a reduced flux
factor convoluted with a gluonic Pomeron with both soft and hard components.

15. Hard Single Diffraction
distributions using (0,0) bin
Prescription based on
calorimeter energy deposition
s = 1800 GeV forward
central
s = 630 GeV forward = Dp p
used to extract  distributions,
fractional energy loss of
diffracted beam particle
  0.2 for s = 630 GeV
Shaded bands indicate
variance due to calorimeter
energy scale uncertainties

16. Double Gaps at 1800 GeV |Jet h| < 1.0, ET > 15 GeV
Gap Region 2.5<|h|<5.2
Demand gap on one side,
and measure multiplicity
on opposite side

17. Double Gaps at 630 GeV |Jet h| < 1.0, ET > 12 GeV
Gap Region 2.5<|h|<5.2
Demand gap on one side,
and measure multiplicity
on opposite side

18. Diffractively Produced W and Z
Electron from W decay, with missing ET
Rapidity gap
Process probes quark
content of Pomeron  
W  e
Z  e+e-
considered
and
require single
interaction to
preserve possible
rapidity gaps
(reduces available
stats considerably)
May expect jets accompanying W or Z
Publication (hep-ex/ ) accepted by Phys. Lett. B

19. Diffractively Produced W’s 
Minimum side
68 of 8724
in (0,0)
nL0 L0
ncal
Central and forward electrons considered  
Minimum side
Peaks in (0,0) bins indicate diffractive W
23 of 3898
in (0,0)
Plot multiplicity in 3<||<5.2 (minimum side)
nL0
ncal L0

20. Diffractively Produced Z’s 
Minimum side
9 of 811
in (0,0)
Plot multiplicity in 3<||<5.2
(minimum side)
nL0 L0
ncal
ncal
Peak in (0,0) bin indicates diffractive Z
ncal

21. Standard W Events Diffractive W Candidates
Compare diffractive W characteristics to all W’s
Standard W Events Diffractive W Candidates
ET=35.2
ET=35.1
Electron ET
ET=36.9
ET=37.1
Good agreement
given lower
diffractive statistics
Missing ET
MT=70.4
MT=72.5
Transverse mass

22. Fraction of diffractively produced W/Z
Diffractive W/Z signals extracted from fits to the 2-D multiplicity distributions,
similar to hard single diffraction dijet analysis
Small correction to fitted signal from residual contamination from multiple interaction
events NOT rejected by single interaction requirement (based on # vertices, L0 timing)
Corrections due to jets misidentified as electrons and Z’s which fake W’s very small
Sample Diffractive Probability Background
All Fluctuates to Data
Central W ( )% x s
Forward W ( )% x s
All W ( – 0.17)% 3 x s
All Z ( )% x s
Opposite
trend compared
to hard diffractive
dijet case
Partonic models don’t describe the entirety of the data (give more Forward than Central)
Forward W isn’t described well by any normalization.
Soft Color seems to do best, but not evolved enough
CDF was 3.8sig signal, significant existence for Z even with few events {

23. W+Jet Rates Jet ET Data Quark Hard Gluon
It is instructive to look at W+Jet rates for rapidity gap
events compared to POMPYT Monte Carlo, since we expect
a high fraction of jet events if the Pomeron is
dominated by the hard gluon NLO process.
Jet ET Data Quark Hard Gluon
>8GeV (10 ± 3)% % %
>15GeV (9 ± 3)% % %
>25GeV (8 ± 3)% % %
The W+Jet rates are consistent with a quark dominated
pomeron and inconsistent with a hard gluon
dominated one.

24. W/Z Cross Section Ratio
RD = (WD ) / ( ZD ) = R*(WD/W)/ (ZD/Z)
where WD/W and ZD/Z are the measured gap fractions from
this analysis
and
R=(W)/ (Z) = ± 0.15(stat) ± 0.20(sys) ± 0.10(NLO)
B. Abbott et al. (D0 Collaboration), Phys. Rev D 61, (2000)
Substituting in these values gives
RD =
This value of RD is somewhat lower than, but consistent with,
the non-diffractive ratio.

25. Diffractive W Boson x
Calculate x = Dp/p for W boson events using calorimeter energy deposition prescription: = Dp p
Sum over all particles in event: those
with largest ET and closest to gap given
highest weight in sum (particles lost
down beam pipe at –h do not contribute
Use only events with rapidity gap
{(0,0) bin} to minimize
non-diffractive background
Correction factor 1.5 ± 0.3 derived from
Monte Carlo used to calculate 
Most events  < 10%, mean is 5%

26. Hard Single Diffraction
Central gap
630 GeV
Forward
630 GeV
All Z
All W
1.0%
Hard Single Diffraction
Central
630 GeV
Central W
Central gap
1800 GeV
Forward
1800 GeV
Forward W
Central
1800 GeV
OK, so it’s right – about 1%
Tomorrow, I’ll have a few words to say about
augmenting this plot at LHC (CMS) energies